In communication network and transmission line using an ATM (Asynchronous Transmission Mode) like broad-band ISDN, video information in variable length digital form is packed into a frame which is called a cell, and has a predetermined fixed bit length. The video information is transmitted, multiplexed/de-multiplexed, and/or exchanged for each cell, so that high speed large capacity communication is obtained with the required flexibility.
In an ATM network which is based upon a statistical multiplex principle, cell loss might occur with some statistics due to possible congestion of network nodes and/or bit errors in cell headers. This causes burst errors or synchronization slips in end-to-end data bit-streams and damages decoded picture quality. Since that error cannot be compensated by conventional forward error correction systems such as BCH or Reed-Solomon coding, some specific consideration is requested so that excellent picture quality is kept in spite of cell loss, when a video signal is encoded with less redundancy through information compression.
A structured packing system has been proposed for that purpose. That structured packing system encodes video signals for each picture block, taking into account the structure of coded video signals, so that a boundary of cells coincides with the data boundary of coded video signals or picture blocks in every predetermined period. The structured packing system has the advantage that the deterioration of picture quality due to cell loss is limited.
However, the prior structured packing system has the disadvantage that some undesirable fill bits which are idle must be inserted in a cell, in order to coincide a boundary of cells with the data boundary of coded video signals in every predetermined period. When we try to shorten the period to coincide the boundaries to get rid of the deterioration in the picture quality, the transmission efficiency is decreased because of an increase in idle fill bits. Therefore, the prior structured packing system has the trade off between picture quality and transmission efficiency.